Catalyst thin layers are used in many applications to promote reactions. For example, they can be used in energetic systems, such as catalyst combustion systems or in sensor systems such as glucose, hydrogen or oxygen detectors, as well as in microsystems such as micro-electro-mechanical-systems (MEMs), LabOn-chips or micro fluidic systems. Catalyst thin layers are more specifically used in the fabrication of catalytic electrodes for fuel cell.
A basic structure of a fuel cell is schematically illustrated in FIG. 1. The fuel cell comprises an electrolytic material 1, that is sandwiched between two electrodes, for example, between a porous anode 2 and a porous cathode 3. An electrochemical reaction occurs between a fuel gas 4 and an oxidant gas 5. A hydrogen cell uses hydrogen as fuel and oxygen (usually from air) as oxidant. Other fuels include hydrocarbons and alcohols, as for example, glucose in abiotic biofuel. Other oxidants include air, chlorine and chlorine dioxide. Fuel cell electrodes may be made of metal, nickel or carbon nanotubes, and are generally coated with a catalyst layer 6 for higher efficiency in ion generation and conductive transfer. The input fuel gas 4 and the oxidant gas 5 flow respectively to the anode 2 and to the cathode 3 through gas supply pathways in plates 7. The input fuel gas 4 and the oxidant gas 5 are catalytically dissociated into ions and electrons in the anode 2 and in the cathode 3.
In solid polymer electrolyte fuel cell also known as proton exchange membrane (PEM) fuel cell, a proton exchange membrane (PEM) 1 constitutes the electrolytic material (FIG. 1). This membrane is sandwiched between the two electrodes, preferably covered by catalyst layers 6. The PEM 1 is proton permeable but constitutes an electrical insulator barrier. This barrier allows the transport of protons from the anode 2 to the cathode 3 through the PEM 1 but forces the electrons to travel around a conductive path to the cathode 3.
Catalyst layers 6 are preferably formed on both surfaces of the PEM 1 to promote electrochemical reactions. The performance data of such a fuel cell depends critically on the quality of the interface between catalyst layers 6 and the PEM 1.
In the prior art, catalyst layers 6 have been incorporated by hot pressing or by ink application directly onto the surface of the PEM 1.
As illustrated in FIG. 2, patent EP-B-0600888 and patent publication U.S. Patent Application No. 2005/0064276 disclose a catalyst layer 6 on a PEM 1 comprising catalyst nano-particles 8 of platinum supported on carbon particles 9 obtained from a homogeneous ink preparation. The latter comprises supported platinum catalyst nano-particles 8 uniformly disperse in a proton conducting material also called ionomer 10. Indeed, the carbon particles 9 of the above-mentioned catalyst layers, are ten to hundred times larger than catalyst metal nano-particles 8. The catalytic sites where the gas reaction takes place are therefore relatively small and, the three-phase interface and the catalyst content are not efficient enough.
Moreover, EP-B-1137090 discloses a method for forming a catalyst layer consisting in sputtering a catalytic metal and a carbon source on a PEM 1 to form, as illustrating in FIG. 3, a nanophase of catalyst nano-particles 8 and nano-sized carbon particles 9. Both catalyst nano-particles 8 and nano-sized carbon particles 9 have a preferred particle size of 2 to 10 nm.
In any case, the catalyst layer contains carbon particles, which have a bad conductivity i.e. a conductivity of less than 104 S/m. Furthermore, the access to the catalytic sites might be difficult.